The present invention is generally related to satellite communications systems and, more particularly, to a constellation of non-geostationary satellites that may be deployed and utilized in a manner that substantially increases global communications satellite capacity and does not interfere with the existing geostationary satellite ring.
Geostationary (xe2x80x9cgeoxe2x80x9d) satellites for telecommunications applications were first proposed many years ago by the author Arthur C. Clark. Today, there are numerous communications systems employing geo satellites for such diverse applications as telephone and data trunking, television distribution, direct-to-home broadcasting, and mobile communications. Geo satellites operate on the physical principle that a satellite, in circular orbit at the proper altitude above the equator, will orbit the earth at the same angular velocity as the earth""s rotation. These satellites therefore, appear to be fixed relative to a point on the earth. This characteristic of geo satellites facilitates their use for communications applications by allowing communications terminals on the earth to simply point their antennas at one position in the sky.
There are however, a number of distinct drawbacks associated with geostationary satellite systems. One major drawback is the high cost of raising a satellite into geo orbit. Geostationary orbits have a radius from the earth center of approximately 36,000 kilometers. Typically, a geo satellite is launched first into an elliptical transfer orbit having an apogee at geostationary altitude, and then its orbit is circularized by using a kick motor to impart the necessary addition momentum to the satellite at apogee. The apogee kick motor, before it is fired, typically weighs as much as the satellite itself, meaning that the launch vehicle must initially launch a payload twice as heavy as the satellite in final orbit. Accordingly, the cost of putting a satellite into the high circular orbit required for geostationary operation is significantly greater than for non-geostationary satellites. The cost associated with deployment of satellites must generally be amortized over the lifetime of the satellite, making use of geo satellites more expensive.
Another problem associated with the altitude at which geo satellites orbit is the delay in the round trip transmission to and from the satellite. For a pair of diverse communications terminals located within the coverage area of a geo satellite, the path length from terminal-to-satellite-to terminal is at least 70,000 kilometers. For the average satellite xe2x80x9chopxe2x80x9d the associated transmission delay is approximately one-quarter of a second. For voice communications by satellite, the delay may not be noticeable to most users, but does make it necessary to use special circuitry for echo control. For data communications, the delay complicates the use of protocols that are predicated on the characteristics of terrestrial circuits.
Other problems arise from the geometry of coverage of geo satellite systems. A geostationary satellite system intended to provide xe2x80x9cglobalxe2x80x9d services would include three geo satellites spaced equal along the equatorial arc at 120-degree intervals. The coverage of each of these satellites describes a circle on the surface of the earth with its center on the equator. At the equator, the coverage areas of two adjacent geo satellites overlap approximately 40 degrees in longitude. However, the overlap decreases as latitude increases, and there are points on the earth, north and south of the coverage areas, from which none of the geo satellites is visible. The lack of coverage is most pronounced at points where the coverage areas intersect, mid-way between satellite orbital locations.
For a geo system, in which the satellites are in orbit above the equator, earth stations in the equatorial regions generally xe2x80x9cseexe2x80x9d the satellites at high elevation angles above the horizon. However, as the latitude of an earth station location increases, the elevation angle to geo satellites from the earth station decreases. For example, elevation angles from ground stations in the United States to geostationary satellites range from 20 to 50 degrees. Low elevation angles can degrade the satellite communications link in several ways. The significant increase in path length through the atmosphere at low elevation angles exacerbates such effects as rain fading, atmospheric absorption and scintillation. For mobile communications systems in particular, low elevation angles increase link degradation due to blockage and multi-path effects.
Because each of the geo satellites only covers one part of the world, some communications links may require more than one satellites hop, or some combined use of satellite and terrestrial transmission facilities to reach their destination. The problem with multiple satellite hops is that for satellites in geostationary orbit, there is a corresponding significant increase in total circuit delay. Of course, multiple satellite hops require an earth station located in view of both satellites that can relay the transmission from one satellite to another.
Direct, inter-satellite links have been proposed as a means for extending the coverage of Geo satellites without the need for such an intermediate earth station. Although the inter-satellite link eliminates the earth station and one round-trip path to the satellite, the benefit is largely offset by the delay incurred in the path between the two orbit satellites. For geo satellites spaced at 120 degrees, the path between satellites is approximately 50,000 kilometers. Moreover, the equipment needed on-board the satellites to implement the inter-satellite link, whether microwave or optical, is complex and expensive. As a result, inter-satellite links have not found extensive application in geo stationary satellites.
Another, and perhaps more significant, problem resulting from the specific geometry of the geo orbit, is the limited availability of orbital positions (or xe2x80x9cslotsxe2x80x9d) along the geostationary orbital arc. The ring of geostationary satellites that has grown up over time generally occupies multiple slots spaced two degrees apart and identified by their longitudinal positions. This arrangement has been adopted internationally to allow for satellite communications with a minimum of interference between adjacent satellites operating in the same frequency bands. The two-degree spacing is achieved by using high gain, directional antennas at the ground stations accessing the satellites. The geo ring around the equator thus provides a total of 180 slots (360 degrees/two degrees per slot). Most of the Geo slots are now occupied, making it difficult to find positions for more geo satellites. Frequency, polarization and beam diversity have been used to multiply capacity, but capacity in the geostationary arc remains limited. Moreover, not all geo orbital positions are equally useful or attractive for various applications.
Various non-geostationary satellite systems have been implemented in the past to overcome some of the drawbacks of geo satellites. An example is the Russian Molniya system, which employed satellites in elliptical 12-hour orbits to provide coverage to the northern latitudes in the Soviet Union. The Iridium and Globalstar systems use satellites in low circular orbits to significantly reduce transmission delay. Generally, non-geostationary systems operate in inclined orbits, and pose a potential for interference with satellites operating at the same frequencies as they cross the geo ring.
In January 1999, an application was filed before the Federal Communications Commission (FCC) by Virtual Geosatellite LLC for the construction of a global broadband satellite communications system based on the teachings of U.S. Pat. Nos. 5,845,206 and 5,957,409, issued to the inventor of the present invention and two other individuals on Dec. 21, 1998 and Sep. 28, 1999, respectively. The system proposed in the FCC application employs three arrays of satellites in elliptical orbits, two arrays covering the northern hemisphere and one covering the southern hemisphere, each array having five 8-hour satellites emulating many of the characteristics of geo satellites. The satellites appear to xe2x80x9changxe2x80x9d in the sky because their angular velocity at or near apogee approximates the rotation rate of the earth. Nine so-called xe2x80x9cactive arcsxe2x80x9d are created with centers located at the apogee points of the satellite orbits. The satellites in each of the three arrays move in a repeating ground track from one active arc to the next, so that there is always one active satellite available in each active arc. Satellites are deactivated between arcs. The active arcs occupy a different portion of the sky than any of the geo satellites located near the equator. As a result, the virtual geo satellites are visible from most parts of the northern and southern hemispheres, but do not interfere with satellites in the geo arc. Even with the prior art virtual geo satellite constellation described above, the present inventor recognizes that capacity issues will continue to become more pressing as communication traffic needs, both in terms of bandwidth and capacity, grow. There will be a need for non-geostationary satellite constellations that provide greater capacity than has already been contemplated in the prior art.
Therefore, it is an object of the present invention to provide a system of satellites that substantially increases global communications satellite capacity without interfering with the existing geostationary satellite ring.
It is another objective of the present invention to provide a global system of communications satellites with higher average elevation angles and lower transmission delay than existing geostationary satellites.
It is a further objective of the present invention to provide a global system of communications satellites with lower construction and launch costs than existing geostationary satellites.
It is yet a further objective of the present invention to provide a global system of communications satellites capable of effectively reusing existing geostationary satellite spectrum allocations.
The above-stated objectives, as well as other objectives, features and advantages, of the present invention will become readily apparent from the following detailed description, which is to be read in conjunction with the appended drawings.
The present invention is directed to a constellation of non-geostationary satellites that may be deployed and utilized in a matter the substantially increases global communications capacity and does not interfere with satellites in the existing geostationary ring around the earth""s equator. A system embodiment includes a ground station, including communications equipment and a steerable antenna, located at a point on the earth, a plurality of satellites in inclined elliptical orbits that forms at least two repeating ground tracks that are displaced from each other in longitude, but all have the same shape. The repeating ground tracks bring the satellites the over the same points on the earth everyday. In the preferred embodiment the satellites have a mean motion of 3, meaning they orbit the earth three times per day, but other integer values of mean motion, such as 2 and 4 have potential applicability.
Each orbiting satellite has communications equipment on board for communicating with ground stations. The communications equipment on each satellite in the constellation is enabled only during a portion of the orbit when the satellite is near apogee, the point in the orbit where the satellite altitude is greatest and the satellite is moving most slowly from the viewpoint of the earth station. In the preferred embodiment, with mean motion 3, each of the satellites is enabled near its apogee for duration of 4 hours, which is 50 percent of total orbit period.
Each of the satellite ground tracks has a number of active arcs corresponding to the portion of the satellite orbits during which the communications equipment on the satellites is enabled to communicate. The orbits of the plurality of satellites are configured such that there are at all times at least two satellites in each active arc enabled to communicate. At the same time, the orbits of the satellites forming each ground track are configured such that the separation between enabled satellites in the same ground track is not less than a predetermined amount considered necessary to prevent interference. Preferably, the satellites in each ground track are equally spaced in mean anomaly to achieve the greatest number of satellites enabled at the same time. In the preferred embodiment, continuous communication at a 50 percent duty cycle requires a minimum of six satellites spaced evenly in mean anomaly. As one satellite of the array leaves an active arc, another satellite enters the active arc to take its place. Adding more arrays of six equally spaced satellites to each ground track in the preferred embodiment creates additional orbital slots. Actually, each group of six satellites, in the preferred embodiment, also provides orbital slots to other positions in the ground track spaced at 120-degree intervals around earth. In the preferred embodiment, the orbital parameters allow up 20 satellites to be placed in each active arc of the ground track while maintaining a minimum angular spacing between satellites of at least 2 degrees.
To avoid potential interference between satellites in different ground tracks, the orbits of the satellites in the two or more ground tracks are also configured such that each satellite enabled to communicate in one of the active arcs is separated by at least a predetermined angle from each of the satellites enabled to communicate in the other ground tracks. In the preferred embodiment, the argument of perigee is adjusted to make the elliptical orbits lean over, allowing the active portions of adjacent ground tracts to be brought close together without interference. The argument of perigee in the present invention preferably ranges from 195 degrees to 345 degrees for apogee is in the northern hemisphere and from 15 degrees to 165 degrees for apogee is in the southern hemisphere.
In another aspect of the invention, each of the satellites in the constellation has an orbital height lower than the height necessary for geostationary orbits. This aspect of the invention has the benefit of reducing satellite size and weight for a given communications capacity, reducing launch requirements, and reducing satellite transmission delay. Also launching into elliptical orbits requires less energy than circular orbits, further reducing launch vehicle costs.
In a further aspect of the present invention, the orbits of satellites are configured such that the portion of the orbits during which communications equipment is enabled, is separated from the earth""s equatorial plane by at least a predetermined amount. This feature avoids potential interference with existing satellites in the geostationary ring and allows the communications frequencies allocated to geostationary satellites to be reused for the non-geostationary constellation of the present invention.
In yet a further aspect of the present invention, each satellite has a power system configured to generate an amount of power less than that required when the communications equipment on the satellite is enabled, and more than that required when the communications equipment is not enabled. The power system can store the excess power generated when the communications equipment is not enabled, and use the stored power to supplement the generated power to meet the requirements of the communications equipment when it is enabled. For the preferred embodiment with a duty cycle of 50 percent, satellite weight saving resulting from this power conservation scheme can be significant.
To minimize perturbation effects caused by the earth""s shape and achieve a stable orbit, the present invention preferably also uses the critical orbital inclination of 63.4 degrees.
The preferred embodiment can accommodate 24 ground tracks having 72 non-interfering active arcs in each hemisphere, or a total of 144 active arcs worldwide. If each arc is filled with a maximum of 20 active satellites, the total number of equivalent non-geostationary satellites slots that the present invention can support is 2880, or 16 times as many as the existing Geo stationary ring, assuming minimum two-degree satellite spacing.